The process of making semiconductors, e.g., integrated-circuit transistors, involves numerous processes carried out under very low pressures. These pressures are maintained in what are commonly referred to as “vacuum chambers.” In general, a vacuum chamber is an enclosure connected to a pumping system, e.g., one including a cryo pump or turbo pump. The pumping system maintains low or extremely low pressures, e.g., 10−8 Torr for a base pressure or 5 mTorr during processing. The pumping system can maintain specified concentrations of selected gases in the chamber. A “vacuum tool” is a device that includes one or more vacuum chamber(s) and facilities to transfer workpieces in and out of the vacuum chamber(s). An example of a vacuum tool, specifically a cluster tool, is the ENDURA physical vapor deposition (PVD) machine made by APPLIED MATERIALS. For example, PVD processes for depositing copper (Cu) and tantalum nitride (Ta(N)) require vacuum, e.g., ˜5 mTorr. Throughout this disclosure, “vacuum” refers to pressures much lower than atmospheric (1 atm =760 Torr), e.g., <20 Torr.
Rate-of-rise (ROR) is one of the simplest tools to help monitor the health of the vacuum system. An ROR curve (or “relapse curve”) can be obtained by pumping the system to the pre-selected pressure (base pressure) and then closing the vacuum valve and monitoring the pressure as a function of time. The ROR curve provides a measure of the gas loads that can be easily compared to a “standard” previously obtained curve for a given system.
The ROR curve can be produced by pumping down for at least ten minutes then closing all valves to isolate the chamber to be tested. No pumping is performed for 2 to 3 minutes of testing. The pressure in the chamber is plotted over time. In an example, <=2000 nTorr/min. is an acceptable rate of rise; more than that indicates a need for corrective action. Pressure increase can result from outgassing from moisture or other materials in the chamber, e.g., materials such as hydrocarbons coating the surface of the chamber or process kits. Pressure increase can also result from leaks between the chamber and the outside atmosphere, or between the chamber and its pumping or other components. For example, a leak in a cutoff valve can leak process gases, e.g., nitrogen (N2) or argon (Ar), into the chamber. Particulates in the valve can mechanically block it from closing fully, e.g., particulates from chemical vapor deposition (CVD) systems (e.g., FIG. 2). Failures to close can also be a result of valve end-of-life. Gas leaks can also result from failures of mass flow controllers (MFCs) upstream of the final cutoff valve.
Many semiconductor fabrication plants (“fabs”) perform ROR testing on each chamber to qualify that chamber before using it to produce silicon wafers. Even if multiple chambers are tested simultaneously, this can take a considerable amount of time, e.g., from tens of minutes to hours per chamber. ROR testing must be repeated periodically, e.g., daily or once every 2 to 3 days, increasing the time consumed. Wafers cannot be run during ROR testing, reducing fab throughput. ROR testing can also not notify operators of failures that occur between ROR tests. Since a 300 mm wafer can cost thousands of dollars, early detection of failures can greatly improve the economic viability of a fab. In addition, the measured ROR curve can reflect a variety of failure modes, not all of which can be distinguished on the basis of ROR testing alone. For example, a pressure increase because of N2 ingress could be a process-gas leak or an outside-air leak. Other pressure increases could be from leaks or outgassing. An ROR failure can therefore require further time-consuming testing to determine the cause of a failure. In some schemes, ROR testing is repeated if a failure is indicated. This can require an additional 30 minute delay and repeated pumpdown. There is, therefore, a need for an improved way of testing chambers.
Various fabs use residual gas analyzers (RGAs) to test chambers. RGAs perform mass spectroscopy on molecules in chambers to determine the composition of those molecules or their partial pressures. Various schemes install an RGA on each process chamber to replace ROR testing. However, this requires a large amount of equipment. There is, therefore, a need for a way of testing multiple chambers with less equipment. Some systems use RGAs on transfer chambers to provide in-situ air leak detection for PVD chambers. However, owing to dynamic pressure changes in short time periods (e.g. less than 10 seconds) during wafer transfer, these systems have performance limitations that can prevent them from being used in place of the ROR test in production. For example, some systems are not sensitive enough to detect leaks in nitritation chambers such as Ta(N) or TiN PVD that involve N2 processing. In addition, these systems do not provide detection of leaks in process chambers attached to the buffer chamber. Moreover, if the process recipe calls for multiple process chambers to be open at once, it can be difficult to determine the atmospheres in each chamber independently. Likewise, pressure transients during wafer moving or interference from other actions by the tool can decrease the accuracy of such measurements. As used herein, “measuring a chamber” can include measuring the pressure in a chamber, partial pressures of various gases, or composition of the atmosphere in a chamber, or testing for or detecting leaks.